Iridium Complexes For Photoinduced Hydrogen Production

The conversion of solar energy to hydrogen is both an opportunity as well as a challenge to solve both the energy crisis and the environmental pollution. In recent years, the three-component photocatalytic systems consisting of a photosensitizer, a homogeneous or heterogeneous water reduction catalyst, and an electron donor sacrificial reductant has drawn much attention because of their promising potential for robust photocatalytic H2production. The most efficient and successful hydrogen evolution system is based on cationic cyclometalated iridium complexes of the general formula [Ir(C^N)2(N^N)]+. However, some significant drawbacks exist in this mult-components system based on Ir(Ⅲ) photosensitizers, that are the high volume fraction of organic solution in reaction solution, poor stability of photosensitizer and so on. In this dissertation, different chemical structures of Ir(Ⅲ) complexes were prepared to obtained a simple, efficient and stable hydrogen evolution reaction system. This dissertation is comprised of the following four parts.(1)A series of zwitterionic iridium(Ⅲ) complexes Ir(N^C)2(Hdcbpy)[HC^N=2-(4-trifluoromethyl)phenylbenzothiazole (1), HC^N=2-(3-trifluoromethyl)phenylbenzothiazole (2), HC^N=2-phenylbenzothiazole (3),2-phenyl-5-(trifluoromethyl)benzothiazole (4),2-(N,N-dimethylamino-4-phenyl)-5-(trifluoromethyl)benzothiazole (5); Hdcbpy=4-carboxy-2,2’-bipyridine-4’-carboxylate)] were prepared. These iridium complexes were used as photosensitizers for photoinduced hydrogen production in water solution together with colloidal Pt as a catalyst and triethanolamine (TEOA) as an electron donor. The highest turnover number of1501was obtained when use complex2as photosensitizer due the contribution of trifluoro methyl on phenyl of C^N ligand. The Ir(Ⅲ) complexes anchor to the TiO2surface via carboxyl groups, enhancing the electron transfer from the excited-state of photosensitizer to water reduction catalyst after the Pt nanoparticles was loaded to TiO2, which improves the TONs of photosensitizers.(2)"Water soluble" MoS2nanoparticle and six iridium complexes [Ir(N^C)2(N^N)]+[HC^N=2-(4-trifluoromeththylphenyl)pyridine, N^N=4-carboxy-2,2’-bipyridine-4’-carboxylate (1); HC^N=Benzo[h]quinoline, N^N=4-carboxy-2,2’-bipyridine-4’-carboxylate (2); HC^N= 2-phenylpyridine, N^N=4,4’-bis(hydroxymethyl)-2,2’-bipyridine (3); HC^N=2-(4-trifluoro meththylphenyl)pyridine, N^N=4,4’-bis(hydroxymethyl)-2,2’-bipyridine (4); HC^N=2-(4-ter-butylphenyl)pyridine, N^N=4,4’-bis(hydroxymethyl)-2,2’-bipyridine (5); HC^N=2-phenylpyridine, N^N=4,4’-diamido-2,2’-bipyridine (6)] were prepared. Photocatalytic hydrogen evolution system was obtained by using these Ir(Ⅲ) complexes as photosensitizers together with MoS2nanoparticle catalyst and a sacrificial electron donor (ascorbic acid or triethanolamine). A higher performance of1or2was observed than other complexes taht can be assigned to the contribution of carboxyl groups, which made Ir(III) complexes anchor to the MoS2surface and enhanced the electron transfer from the photosensitizer to MoS2. The catalytic activity of MoS2increased with the decreasing size or better dispersion ability of MoS2. In optimization conditions, the highest TON based on MoS2was up to3124in1-MoS2-TEOA system, which giving an AQY of12.4%under400nm light irradiation. These hydrogen production systems show poor lifetime which can be assigned to the decomposition of Ir(Ⅲ) photosensitizer.(3)A series of heteroleptic neutral tri-cyclometalated iridium complexes Ir(thpy)2(C^N)[thpy=2,2’-thienylpyridine; HC^N=2-phenylbenzothiazole (1),2-(4-trifluoromethyl)phenyl benzothiazole (2), HC^N=2-(3-trifluoromethyl)phenylbenzothiazole (3),2-phenyl-5-(trifluoromethyl)benzothiazole (4)] were prepared. Photocatalytic hydrogen production system was obtained via illumination of aqueous solutions consisting of these iridium complexes photosensitizer,[Co(bpy)3]Cl2catalyst and TEOA. Complex1achieved the highest activity for artificial photosynthesis with348turnover number when the irradiation time was prolonged to72h, and the highest apparent quanta efficiency (AQY) was up to1.62%under350nm light irradiation. The decay of hydrogen evolution rate was assigned to the decomposition of catalyst, and the activity can be regenerated once again after addition of cobalt-based catalyst. Two cationic iridium complexes,[Ir(thpy)2(dtb-bpy)](PF6) and [Ir(ppy)2(dtb-bpy)](PF6)(dtbpy=4,4’-di-tert-butyl-2,2’-dipyridyl) were prepared as reference photosensitizers. The lifetime of photocatalytic system based on these neutral iridium compounds were superior than that based on cationic iridium complexes, which is primarily attributed to the contribution of both an increasing Ir-C sigma bond and an oxidative quenching process of the exticed-state photosensitizer.(4)A series of homoleptic tris-cyclometalated iridium complexes of the general formula [Ir(C^N)3][HC^N-2-phenylpyridine (1), HC^N=2-(4-trifluoromeththyl)phenylpyridine (2), HC^N=2-(4-n-butylphenyl)pyridine (3)] were synthesized. Photocatalytic hydrogen production system was obtained use these Ir(Ⅲ) complexes as photosensitizers together with [Rh(dtb-bpy)3](PF6)3as a water reduction catalyst and triethanolamine (TEOA) as a sacrificial electron donor. These systems maintain its activity more than ca.72hours with a highest total turnover number of3040based on the complex1, and the highest AQY was up to2.59%under380nm light irradiation. The photocatalytic performance and stability of these homoleptic tris-cyclometalated iridium compounds are superior than that of a cationic iridium complex [Ir(ppy)2(bpy)](PF6)(4) as a reference.